The present invention relates to iron-nickel alloy strips that can be used in particular for the manufacture of support grids for integrated circuits in a broad range of electronic components such as static memories, dynamic or programmable memories, and microprocessors.
Electronic components contain integrated circuits manufactured from silicon dice engraved and glued onto grids intended in particular to form connection pads of these integrated circuits. These grids must be made of an alloy having multiple properties that make them compatible with the silicon dice known as “electronic chips” and permitting the manufacture of components under good conditions. In particular, the alloy must have a coefficient of thermal expansion slightly greater than that of silicon, in order to maintain the component in compression during the cooling phases of the thermal cycles of the method for manufacture of the component and during use of same, in order to avoid the development of large mechanical stresses.
In addition, the alloy must have sufficiently high tensile strength Rm that the connection pads do not become deformed during manipulations by means of robots. However, it must also have good shapability, that is, on the one hand sufficient ductility characterized by total elongation greater than 5% and on the other hand a ratio between the yield strength Rp0.2% and Young's modulus E that is sufficiently low to reduce elastic recovery during bending of the pads. All of these characteristics make it possible to avoid any damage to the pads while they are being shaped.
Finally, the obtained strip as well as the manufactured pads must have the greatest possible dimensional stability after cutting, but also throughout the different thermal cycles of the manufacturing method and during use of the strip.
This dimensional stability is manifested by small residual stresses in the strip, to ensure coplanarity of the internal and external pads of the grids, but also by small contraction during the heat treatments used for manufacture of components. It is therefore estimated that the contraction value of a strip having a length of 180 mm and subjected to a heating cycle at 500° C. for 4 minutes must never exceed 15 μm, that is, to a deformation λr smaller than 8.10−3%, and preferably smaller than 4.10−3% for the most exacting components.
Traditionally, the support grids of integrated circuits are made of N42® alloy, which is an iron-nickel alloy containing approximately 41% of nickel. The strips used are obtained by a standard manufacturing method comprising a series of cold-rolling operations with intermediate recrystallization annealing phases, the last operation being a cold-rolling in which the degree of cold work (or percentage thickness reduction) makes it possible to adjust the mechanical characteristics and especially the Rm of the future grid. However, the obtained strips have inadequate characteristics in terms of bendability but also of dimensional stability, and more particularly in terms of contraction. In fact, contractions as severe as 40.10−3% and not going below 6.10−3% are commonly observed.
In order to remedy these disadvantages, it is common practice to use strips that have been subjected to a supplementary final heat treatment known as stress relief annealing, at a temperature very much lower than the recrystallization temperature, in order to reduce the residual stresses and the contraction without significantly lowering the tensile strength. However, this treatment is not capable of reducing the contraction below 5.10−3% or of reducing the totality of the residual stresses, thus resulting in dimensional stability problems that can be observed in grids dedicated to the most exacting applications.